This application claims the priority of Korean Patent Application Nos. 2003-2085 and 2003-23057, filed on 13 Jan. 2003 and 11 Apr. 2003, respectively, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to reducing power consumption in stations on a wireless LAN based on the IEEE 802.11 standard.
2. Description of the Related Art
According to the IEEE 802.11 standard (“standard”), a basic service set (BSS) including an access point (AP) is referred to as an infrastructure mode, and a BSS without APs is referred to as an ad-hoc mode. A collection of stations included in an ad-hoc mode is referred to as an independent BSS (IBSS). Among functions at a media access control (MAC) protocol, a distributed coordination function (DCF) and an enhanced DCF (EDCF), which provides a quality of service (QOS), are executed in only an ad-hoc mode. In the DCF and EDCF, a MAC protocol data unit (MPDU) is transmitted through contention. The period during which the MPDU is transmitted through contention is referred to as a contention period (CP). An infrastructure mode includes all of the functions practiced in an ad-hoc mode, such as, a point coordination function (PCF) and a hybrid coordination function (HCF), which provides a QOS. These functions are implemented according to a contention way and a polling way. The period during which an MPDU is transmitted through polling is referred to as a contention free period (CFP).
FIG. 1 shows a conventional delivery traffic indication map (DTIM). A point coordinator makes a polling list on the basis of a Contention Free-pollable (CF-pollable) subfield and a Contention Free-poll (CF-poll) request subfield, which are included in an association request frame or a reassociation request frame. If each station receives a beacon frame depending on a DTIM, it starts a point coordination function.
FIG. 2 shows a mechanism of a conventional PCF. During a CFP, stations registered in a polling list wait for being polled by a point coordinator. The point coordinator polls the stations on the polling list in an Association Identity-increasing (AID-increasing) order. The CFP may be shortened if a beacon frame is delayed due to channel busyness. Polled stations transmit data to the point coordinator. Unpolled stations wait for their turns while keeping an active state, in which the entire power is supplied, in order to receive a poll frame. Hence, in the conventional PCF mechanism, even stations not transmitting data at present are in an active state just for a moment to receive a poll frame, thus consuming much unnecessary power. The stations are generally mobile stations. In this case, the stations must be small and light, and accordingly, power consumption in the stations must be considered carefully.